1. Field of the Invention
The invention relates to an image reading apparatus which is used as an image reading unit of a copying apparatus or a facsimile or which is connected to a terminal of a personal computer or the like and is used.
2. Related Background Art
In an image reading apparatus, a phenomenon such that even if the same original is read, its output (reading density) differ from each other occurs due to a cause such as fluctuation per hour in a light emission amount of a light source, fluctuation in association with an increase in temperature, fluctuation in sensitivity from pixel to pixel of a photoelectric converting unit, or the like.
To prevent a deterioration in image quality in association with the fluctuation of the reading density, hitherto, white reference data is obtained by scanning a white reference plate prior to reading an original image and a shading compensation of the original image is performed on the basis of the white reference data.
As an apparatus for performing the shading compensation, for example, there has been proposed an image input apparatus (refer to JP-A-61-71764) comprising: a circuit for A/D converting an image signal obtained by scanning a white reference plate a plurality of number of times, accumulating A/D converted image signals every pixel, and storing an accumulation result; and a circuit for calculating a shading compensation coefficient from the accumulated value and storing it, wherein the shading compensation coefficient is obtained with low noises and at a high speed. There has also been proposed an image reading apparatus (refer to JP-A-63-1249) for discriminating an operating life of a light source or a white reference plate by comparing the maximum value of an image signal obtained by scanning a white reference plate by a plurality of lines with a preset threshold value, or the like.
There has also been proposed a shading compensating apparatus (refer to JP-A-1-132275) in which a carriage on which an image sensor, a light source, and the like are mounted is moved while reflection light from a white reference plate is being read, thereby suppressing an influence of a change in partial reflectance on the reference plate.
As mentioned above, the conventionally proposed techniques have a purpose to obtain a shading coefficient of small noises, a purpose to discriminate an operating life of the light source or white reference plate based on the white reference data obtained by reading the white reference plate, and a purpose to reduce the influence of the difference of the partial reflectance when the white reference plate is read.
In the conventional shading compensation, explanation will be made by using a facsimile apparatus 1 in FIG. 6. Prior to reading an original 2, generally, white reference data is obtained by irradiating light from a light source 3 to a white reference plate 4, and the shading compensation is performed on the basis of the white reference data.
That is, in the facsimile apparatus 1, prior to reading the original 2, the light is irradiated from the light source 3 to the white reference plate 4, the reflection light is reflected by a mirror 5 and is introduced into an image sensor 7 through a lens 6. The light is photoelectrically converted by the image sensor 7 and transmitted to an A/D converter 9 through an amplifier 8. The maximum value of the image signal of one line which is inputted to the A/D converter 9 is detected and held by a peak hold circuit 10. On the basis of the peak value detected and held by the peak hold circuit 10 as a reference, the A/D converter 9 quantizes the image signal which was amplified by the amplifier 8 and inputted. The quantized signal is stored as white reference data into a line buffer 12 through a shading compensation circuit 11. (A series of those operations is hereinafter referred to as a shading operation).
When the original 2 is read, the light is irradiated from the light source 3 to the original 2 conveyed on a guide 13. The reflection light is reflected by the mirror 5 and is introduced into the image sensor 7 through the lens 6. The image signal photoelectrically converted by the image sensor 7 is transferred to the A/D converter 9 through the amplifier 8. In this instance, the maximum value of the image signal of one line is detected by the peak hold circuit 10. The maximum value (peak value) is used as a reference, the image signal is quantized by the A/D converter 9, and the quantized signal is outputted to the shading compensation circuit 11.
When the white reference plate 4 is read, a system control unit 14 sequentially reads out the white reference data stored in the line buffer 12 every pixel in response to the image signal that is inputted from the A/D converter 9. The read-out white reference data is sent to the shading compensation circuit 11. The image signal inputted from the A/D converter 9 to the shading compensation circuit 11 is subjected to a shading compensation calculation on the basis of the white reference data and a shading compensated signal is outputted to a digital image processing circuit 15. Thus, the digital image processing circuit 15 performs various image processes to the shading compensated image signal.
When the image reading apparatus of the conventional facsimile apparatus 1 or the like obtains the white reference data by once reading the white reference plate 4 before the original 2 is read, the apparatus subsequently executes the reading operation of the original 2 and shading-compensates the read image signal on the basis of the white reference data (hereinafter, the above series of operations is referred to as an image reading operation).
In such a conventional image reading apparatus, however, the white reference plate is certainly scanned once prior to reading one original and the white reference data is obtained. Therefore, when a number of originals exist, a time for the shading operation occurs every time. There is, consequently, a problem such that in case of reading a number of originals, a time for the total image reading operation in which the shading operation, the image reading operation, and the other operations necessary for the image formation or the like are combined becomes long.
To prevent it, a method of reducing the shading operation is also considered. However, for example, when a number of originals are read, if the reflection light from the white reference plate is read out and white reference data is obtained at only the first time, when a change in light amount of a light source by which the first original can be accurately read or the like occurs, a proper light amount compensation is not performed in case of reading the originals of the latter half of a number of originals. There occurs a problem such that when the reduction of the light amount occurs, the read data becomes dense and, contrarily, when the light amount increases, the read data becomes light.
The invention is made in consideration of the above problems and it is an object of the invention to provide an image reading apparatus which can optimize an image reading speed and a reading image quality.
Another object of the invention is to provide an image reading apparatus comprising:
a light source for illuminating an original;
a white member which is illuminated by a light emitted from the light source;
a photoelectric converting element for receiving reflection light from the original and reflection light from the white member;
memory means for storing a reference output of the photoelectric converting element which is obtained by receiving the reflection light from the white member;
shading compensation means for compensating an output of the photoelectric converting element which is obtained by receiving the reflection light from the original in accordance with the reference output; and
rewriting means for rewriting the reference output to be stored in the memory means,
wherein the rewriting means rewrites the reference output at a timing according to a light amount fluctuation of the light source.
Still another object of the invention is to provide an image reading apparatus comprising:
a light source for illuminating an original;
a white member which is illuminated by a light emitted from the light source;
a photoelectric converting element for receiving reflection light from the original and reflection light from the white member;
memory means for storing a reference output of the photoelectric converting element which is obtained by receiving the reflection light from the white member;
shading compensation means for compensating an output of the photoelectric converting element which is obtained by receiving the reflection light from the original in accordance with the reference output; and
rewriting means for rewriting the reference output to be stored in the memory means,
wherein the rewriting means rewrites the reference output at a timing according to a lightening time of the light source.
Still another object of the invention is to provide an image reading apparatus comprising:
a light source for illuminating an original;
a white member which is illuminated by a light emitted from the light source;
a photoelectric converting element for receiving reflection light from the original and reflection light from the white member;
memory means for storing a reference output of the photoelectric converting element which is obtained by receiving the reflection light from the white member;
shading compensation means for compensating an output of the photoelectric converting element which is obtained by receiving the reflection light from the original in accordance with the reference output; and
rewriting means for rewriting the reference output to be stored in the memory means, in which in case of continuously reading a plurality of originals, the rewriting means rewrites the reference output every time the number of the read originals amounts to predetermined numbers,
wherein a number of originals to be read from a rewriting to a rewriting of the reference output by the rewriting means gradually increases.
Still another object of the invention is to provide an image reading apparatus comprising:
a light source for illuminating an original;
a white member which is illuminated by a light emitted from the light source;
a photoelectric converting element for receiving reflection light from the original and reflection light from the white member;
memory means for storing a reference output of the photoelectric converting element which is obtained by receiving the reflection light from the white member;
shading compensation means for compensating an output of the photoelectric converting element which is obtained by receiving the reflection light from the original in accordance with the reference output; and
rewriting means for rewriting the reference output to be stored in the memory means,
wherein the apparatus is selectably set in one of a first image reading mode and a second image reading mode in which the number of times of rewriting of the reference output when a plurality of originals are read is smaller than that in the first image reading mode.
The above and other objects and features of the present invention will become apparent from the following detailed description with reference to the accompanying drawings.